1. Field of the Invention
The invention generally relates to DC (Direct Current) motors used in various applications, such as hard disk drive motors, cooling fans, drive motors for appliances, etc.
2. Description of the Related Art
An electric motor uses electrical energy to convert to mechanical energy. Electric motors are used in a large number of applications, including a number of different household appliances, pumps, cooling fans, etc. Motors can generally be classified as either alternating current (AC) motors or direct current (DC) motors.
Motors generally include a rotor, which is the non-stationary (moving) part of the motor, and a stator, which is the stationary part of the motor. The stator generally operates as a field magnet (e.g., electromagnet), interacting with an armature to induce motion in the rotor. The wires and magnetic field of the motor (typically in the stator) are arranged so that a torque is developed about the rotor's axis, causing rotation of the rotor. A motor typically also includes a commutator, which is an electrical switch that periodically reverses the current direction in the electric motor, helping to induce motion in the rotor. The armature carries current in the motor and is generally oriented normal to the magnetic field and the torque being generated. The purpose of the armature is to carry current crossing the magnetic field, thus creating shaft torque in the motor and to generate an electromotive force (“EMF”).
In a typical brushed DC motor, the rotor comprises one or more coils of wire wound around a shaft. Brushes are used to make mechanical contact with a set of electrical contacts (called the commutator) on the rotor, forming an electrical circuit between the DC electrical source and the armature coil-windings. As the armature rotates on axis, the stationary brushes come into contact with different sections of the rotating commutator. The commutator and brush system form a set of electrical switches, each firing in sequence, such that electrical-power always flows through the armature coil closest to the stationary stator (permanent magnet). Thus an electrical power source is connected to the rotor coil, causing current to flow and producing electromagnetism. Brushes are used to press against the commutator on the rotor and provide current to the rotating shaft. The commutator causes the current in the coils to be switched as the rotor turns, keeping the magnetic poles of the rotor from ever fully aligning with the magnetic poles of the stator field, hence maintaining the rotation of the rotor. The use of brushes creates friction in the motor and leads to maintenance issues and reduced efficiency.
In a brushless DC motor design, the commutator/brushgear assembly (which is effectively a mechanical “rotating switch”) is replaced by an external electronic switch synchronized to the rotor's position. Brushless DC motors thus have an electronically controlled commutation system, instead of a mechanical commutation system based on brushes. In a brushless DC motor, the electromagnets do not move, but rather the permanent magnets rotate and the armature remains static. This avoids the problem of having to transfer current to the moving armature. Brushless DC motors offer a number of advantages over brushed DC motors, including higher efficiency and reliability, reduced noise, longer lifetime (no brush erosion), elimination of ionizing sparks from the commutator, and overall reduction of electromagnetic interference (EMI).
One technique used to reduce the power required in some applications has been the introduction of Three Phase Brushless Motors. The typical configuration for these motors is shown in FIG. 1. The drive electronics for these motors typically rely on Hall elements (Hall effect sensors) to detect the absolute position of the rotor at all times, and switch drive transistors to maintain motor rotation. A Hall effect sensor is a transducer that varies its output voltage in response to changes in magnetic field. The motors are often electrically connected in a “Y” configuration, so named due to the resemblance to the letter “Y”. The common point for the three coils is connected to the electrical source, and the drive electronics switch the drive transistors to maintain the rotating electro-magnetic field required to turn the motor.
A second method requires the use of six (6) drive transistors. In this configuration, one high- and low-side pair are on at any point in time, completing the electrical circuit through two of the three legs of the motor. Using the un-energized coil as a magnetic sensor to determine the rotor position is known as Back Electro-Motive Force (BEMF) detection. The motivation for this technique is the elimination of the relatively expensive Hall elements and associated electronics. BEMF commutation techniques have successfully been applied to a wide-range of motors.
Prior art solutions typically use a brute-force method to drive the motor coils during start-up, which may last several seconds, and may draw several times the normal operating current. The period of time when this occurs is referred to in the literature as the Forced Commutation phase of spin-up. This is one of the drawbacks of the BEMF commutation method. Until the motor spins sufficiently fast enough to generate a BEMF signal, the motor is typically driven open loop, at a pre-determined frequency and pulse width modulation (PWM) duty cycle, putting undue stress on the motor components. The currents used are often sufficient to damage the motor windings, and without a feedback method, a timer must expire before the damaging condition can be detected, and corrected. In some solutions, there is no provision for this event, and the motor will continue to drive to destruction.
Therefore, improvements in motor design and operation are desired.
One issue that arises when using BEMF detection is that if the rotor is not moving, there is no BEMF to be detected. This means there must be a special technique for inducing rotation of the rotor until the rotational speed is sufficient to detect a BEMF signal. Since there is no feedback to determine the exact position of the rotor, the stator coils must be energized such that the rotor moves to a known, predictable location. This is essential to starting the motor with high reliability.
Open literature describes two techniques for aligning the rotor. The first attempts to lock the commutation frequency to a reference oscillator from the initial rotation. The second method uses a ramping PWM duty cycle or ramping linear voltage to align the rotor, followed by a delay time to ensure the rotor is at the pre-determined position. This position varies from 0-30 degrees offset from the commutation point. Both of these techniques leave much to be desired.
Most of these solutions reach 100% PWM duty cycle (or maximum voltage for linear applications) for an extended period of time to align the motor. This is an energy intensive way to align a rotor, and has the potential to damage motor windings. It is also known that slight movements of the rotor may occur when maximum voltage is applied to the coils, then switched off. Some solutions implement complex control loops in order to align the rotor. Many solutions rely on the classic Park and Clarke transforms to determine position, and require microcontrollers to evaluate the mathematics. Improvements in rotor alignment techniques that address some or all of these problems would be desirable.
In addition, motor braking techniques typically do not consider the possibility of placing the rotor in a position conducive to the next start-up sequence. The industry standard solution is to perform one of two operations: either to connect all three phases to the common point, or to connect all three phases to ground. This makes the restart sequence more complicated and more prone to stalling conditions, as the exact rotor location in unknown. Improvements in motor braking techniques would therefore also be desirable.
Prior art solutions typically use complex control loops, such as the phase-locked-loop (PLL) to align and start a three phase motor. The PLL approach assumes that the BEMF signal will be present from the initial rotation. For small motors, the initial slope may be as low as 10 mV in a 5V environment, where a large current and voltage spike would corrupt the signal. PLL solutions also lock the back EMF and track the increasing frequency of the PLL as the speed of the motor increases. Open literature describes the use of PLL techniques for motor speed control by placing a linear control system with a speed setpoint as a reference input and a tachometer to measure the motor's rotational velocity. During normal operation, the phase frequency detector will be in a nonlinear regime as the motor speed is ramped to different setpoints. This make the PLL solutions to have a phase error between the output phase and the reference phase.
Therefore, improved methods are desired for aligning and starting a motor.
When cooling any computing platform, power is necessarily consumed to remove heat produced by other components in the system. Traditionally, this has not been a large concern, as the platforms consumed much more power than the fan used in removing the heat. As the power consumption of all platforms is reduced, the cooling system consumes power that could either be used to extend battery life in laptops, or to reduce the carbon footprint of server systems. Therefore, improvements in motors used in cooling systems are also desired.